This invention relates to a method for producing rotary dressers. Known manufacturing processes of this type are primarily divided into two types; electro-plating process and sintering-infiltration process.
The former process utilizes an electrically deposited metal layer for cementing super-abrasive grit such as diamond or cubic boron nitride (CBN). There are reverse electro-plating type and single-layer surface type of this process. On the other hand, the latter process utilizes a sintered metal to cement super-abrasive grit such as diamond or CBN. Among the rotary dressers made by this process, there are single- and multi-layer grit types. This invention relates to the latter type process, i.e., a method utilizing sintering-infiltration. An outline of a conventional process utilizing this method will now be explained.
FIG. 1 shows a negative mold 1 made of, for instance, graphite. Super-abrasive grit 3 such as diamond or CBN are set on the mold surface 2 (the inner diameter surface). A core piece 4 is then placed in the assembly, as is shown in FIG. 2, and a metallic powder 5 is filled in the space between the mold surface 2 of the negative mold 1 and the outer diameter of core piece 4.
Infiltrant material 6 is put on the metal powder 5, and the whole construction is heated to a temperature above the melting point of the infiltrant. The infiltrant thus melts down and penetrates into the voids of the metal powder filling 5. After the cooling and solidifying of the infiltrant, an infiltrated solid body cementing the super-abrasive grit 3 and metal powder 5 is obtained. The negative mold 1 and core piece 4 are then removed, and the infiltrated solid body is machined to the specified dimensions and shape. Thus a rotary dresser as shown in FIG. 3 having super-abrasive surface 7 is obtained.
An expensive metal powder such as tungsten is generally used as the above-mentioned metal powder 5, because the metal powder must be highly heat- and wear-resistant. In order to minimize the use of metal powder 5, in some cases, a core piece 8 generally made of iron or iron alloy is placed in the center of negative mold 1, as is shown in FIG. 4, so that the space between the mold surface 2 of negative mold 1 and the outer diameter of core piece 8 becomes as narrow as possible. The tungsten is filled in said space, treated by the above-mentioned infiltration process, and finally finished to make the core piece 8 a part of the rotary dresser. FIG. 5 shows an example of the rotary dresser made by the above-mentioned method.
As is shown in FIG. 5, the rotary dresser has a sintered metal powder such as tungsten 9 formed on the outer diameter of the core piece 8, and a super-abrasive surface 7. As is explained above, the use of a core piece of iron or iron alloy makes it possible to minimize the use of expensive tungsten powder. However, where the core piece is used, the strength of the junction between the core piece 8 and the sintered metal powder 9 is equal only to the brazing strength between the core piece 8 and sintered metal powder 9, which are essentially different metallic materials. If the infiltration of infiltrant 6 is insufficient or if the wetting of the infiltrant 6 on the core piece 8 is poor, poor brazing is observed; and thus the strength of the junction between core piece 8 and the sintered metal powder 9 becomes poor. It is not seldom that the separation of core piece 8 and sintered metal powder 9 is observed. The big difference in the thermal expansion coefficients of the two different materials, i.e., the core piece 8 and the sintered metal powder 9, causes large internal stresses during cooling after infiltration and results in a strain on the super-abrasive surface 7, which translates into a deterioration in the accuracy of the rotary dresser.